In a fuel injection system, (unlike a carburetor controlled system) the fuel delivery system and the air delivery system of an internal combustion engine are separated. As a result, explicit devices must be provided to determine how much fuel to deliver and to provide the actuating and regulating mechanism to deliver it. Since the amount of fuel needed by an engine depends on the amount of air being used (or vice-versa), any fuel-injected engine needs some form of measurement of the actual airflow through the engine.
Thus, in copending application Ser. No. 228,973, filed Jan. 27, 1981, which is assigned to the assignee of this application, an electronically controlled fuel injection system for a spark ignition internal combustion engine is disclosed wherein airflow rate is controlled automatically to provide a proper ratio with the fuel flow rate established by an operator, as by depression of an accelerator pedal. A computer is utilized to calculate the optimum airflow rate using the fuel flow command input and various correction information derived from certain engine parameters. The calculated airflow rate is applied by controlling the angular position of a rotatable throttle plate within an air passage to the engine.
In some other fuel injection systems, the fuel flow rate is similarly determined from an airflow command input.
In the operation of a vehicle, it may be necessary or desirable at times to operate it with a full or wide-open throttle. With fuel injection engines of either type described, this presents some problems.
The two most common means of measuring the engine's airflow in fuel injected engines are a flow meter in series with the inlet air and a hot-wire meter similarly placed. Since there is always some pressure drop across the meter, the maximum power of the engine is diminished at wide-open throttle by the presence of either type of meter.
This diminution in engine power is due to a type of "insertion loss", due to the fact that the act of making the measurement also affects the system being measured.
In an attempt to minimize insertion loss, many fuel injected systems measure airflow indirectly by measuring the pressure drop across the throttle, or by measuring the absolute pressure in the intake manifold. In systems that derive the airflow measurement from the pressure drop across the throttle, that pressure drop becomes nearly zero when the throttle goes to wide open, and the measurement of airflow by the pressure drop becomes meaningless, because the signal available is smaller than the precision limit of the instrument. This type of measurement therefore threatens the loss of engine control, and in fuel injected systems wherein the amount of fuel to be injected is calculated from airflow, airflow measurement is required for the controller to know how much fuel to give the engine. Accurate airflow measurement at wide-open throttle is just as vital in systems controlling airflow relative to fuel flow.
There are several possible strategies for overcoming this problem. One would be to use the engine speed to predict the amount of air the engine will use. However, this approach is generally impractical because there are significant differences in volumetric efficiency among engines as manufactured. Also, as engines age, their volumetric efficiency changes, and thus, the prediction may not be as good as is necessary for desired engine performance. Another possible solution is to prevent the throttle from opening beyond that point at which sufficient precision is still available in the measurement of pressure drop to give a meaningful airflow measurement. But when the throttle is never able to reach its widest opening, the maximum power available from the engine is reduced.
It is, therefore, a general object of the present invention to solve the aforesaid problems and to provide a method for operating a fuel injection internal combustion engine automatically and efficiently with the proper air-fuel ratio at the wide-open throttle position.
Another object of the invention is to provide such a method for wide-open throttle operation of a fuel injected engine that enables maximum power to be attained and avoids the necessity of relying on a priori engine performance data.
Still another object of the invention is to provide a system for enabling wide-open throttle operation of a fuel-injected engine that uses a controllable throttle actuated by signals from an electronic computer within the system.